The Flow of Energy: Higher Trophic Levels
The trout, in turn, must consume 90,000 frogs, that must consume 27 million
grasshoppers that live off of 1,000 tons of grass.
-- G. Tyler Miller, Jr., American Chemist (1971)
In this lesson, we will answer the following questions:
Jump to: [Introduction] [Energy Transfer] [Fox and Hare Example] [Pyramids Models] [Human Energy Consumption] [Summary]
Most of you are now familiar with the concept of the trophic level (see Figure 1). It is simply a feeding level, as often represented in a food chain or food web. Primary producers comprise the bottom trophic level, followed by primary consumers (herbivores), then secondary consumers (carnivores feeding on herbivores), and so on. When we talk of moving "up" the food chain, we are speaking figuratively and mean that we move from plants to herbivores to carnivores. This does not take into account decomposers and detritivores (organisms that feed on dead organic matter), which make up their own, highly important trophic pathways.
Figure 1: Trophic levels.
C6H12O6 + 6 O2 -------- 6 CO2 + 6 H2O
In the process, metabolic work is done and energy in chemical bonds is converted to heat energy. If NPP was not consumed, it would pile up somewhere. Usually this doesn't happen, but during periods of earth history such as the Carboniferous and Pennsylvanian, enormous amounts of NPP in excess of consumption accumulated in swamps. It was buried and compressed to form the coal and oil deposits that we mine today. When we burn these deposits (same chemical reaction as above except that there is greater energy produced) we release the energy to drive the machines of industry, and of course the CO2 goes into the atmosphere as a greenhouse gas. This is the situation that we have today, where the excess CO2 from burning these deposits (past excess NPP) is going into the atmosphere and building up over time.
But let's get back to an ecosystem that is balanced, or in "steady state" ("equilibrium"), where annual total respiration balances annual total GPP. As energy passes from trophic level to trophic level, the following rules apply:
An Example: The Fox and the HareTo understand these rules, we must examine what happens to energy within a food chain. Suppose we have some amount of plant matter consumed by hares, and the hares are in turn consumed by foxes. The following diagram (Figure 2) illustrates how this works in terms of the energy losses at each level.
The hare uses a significant fraction of the assimilated energy just being a hare -- maintaining a high, constant body temperature, synthesizing proteins, and hopping about. This energy used (lost) is attributed to cellular respiration. The remainder goes into making more hare biomass by growth and reproduction. The conversion of assimilated energy into new tissue is termed secondary production in consumers, and it is conceptually the same as the primary production or NPP of plants. In our example, the secondary production of the hare is the energy available to foxes who eat the hares for their needs. Clearly, because of all of the energy costs of hares engaged in normal metabolic activities, the energy available to foxes is much less than the energy available to hares.
Just as we calculated the assimilation efficiency above, we can also calculate the net production efficiency for any organism. This efficiency is equal to the production divided by the assimilation for animals, or the NPP divided by the GPP for plants. The "production" here refers to growth plus reproduction. In equation form, we have net production efficiency = (production / assimilation), or for plants = (NPP / GPP). These ratios measure the efficiency with which an organism converts assimilated energy into primary or secondary production.
These efficiencies vary among organisms, largely due to widely differing metabolic requirements. For instance, on average vertebrates use about 98% of assimilated energy for metabolism, leaving only 2% for growth and reproduction. On average, invertebrates use only ~80% of assimilated energy for metabolism, and thus exhibit greater net production efficiency (~20%) than do vertebrates. Plants have the greatest net production efficiencies, which range from 30-85%. The reason that some organisms have such low net production efficiencies is that they are homeotherms, or animals that maintain a constant internal body temperature. This requires much more energy than is used by poikilotherms, which are organisms that do not regulate their temperatures internally.
Just as we can build our understanding of a system from the individual to the population to the community, we can now examine whole trophic levels by calculating ecological efficiencies. Ecological efficiency is defined as the energy supply available to trophic level N + 1, divided by the energy consumed by trophic level N. You might think of it as the efficiency of hares at converting plants into fox food. In equation form for our example, the ecological efficiency = (fox production / hare production).
Thinking about ecological efficiency brings us back to our first rule for the transfer of energy through trophic levels and up the food chain. In general, only about 10% of the energy consumed by one level is available to the next. For example, If hares consumed 1000 kcal of plant energy, they might only be able to form 100 kcal of new hare tissue. For the hare population to be in steady state (neither increasing nor decreasing), each year's consumption of hares by foxes should roughly equal each year's production of new hare biomass. So the foxes consume about 100 kcal of hare biomass, and convert perhaps 10 kcal into new fox biomass. In fact, this ecological efficiency is quite variable, with homeotherms averaging 1- 5% and poikilotherms averaging 5-15%. The overall loss of energy from lower to higher trophic levels is important in setting the absolute number of trophic levels that any ecosystem can contain.
From this understanding, it should be obvious that the mass of foxes should be less than the mass of hares, and the mass of hares less than the mass of plants. Generally this is true, and we can represent this concept visually by constructing a pyramid of biomass for any ecosystem (see Figure 3).
Figure 3. A pyramid of biomass showing producers and consumers.
We could also construct a pyramid of numbers, which as its name implies represents the number of organisms in each trophic level (see Figure 4a). For the oceans as shown in Figure 4, the bottom level would be quite large, due to the enormous number of small algae. For other ecosystems, the pyramid of numbers might be inverted: for instance, if a forest's plant community was composed of only a handful of very large trees, and yet there were many millions of insect grazers which ate the plant material.
Just as with the inverted pyramid of numbers, in some rare exceptions, there could be an inverted pyramid of biomass, where the biomass of the lower trophic level is less than the biomass of the next higher trophic level. The oceans are such an exception because at any point in time the total amount of biomass in microscopic algae is small. Thus a pyramid of biomass for the oceans can appear inverted (see Figure 4b). You should now ask "how can that be?" If the amount of energy in biomass at one level sets the limit of energy in biomass at the next level, as was the case with the hares and foxes, how can you have less energy at the lower trophic level? This is a good question, and can be answered by considering, as we discussed in the last lecture, the all important aspect of "time". Even though the biomass may be small, the RATE at which new biomass is produced may be very large. Thus over time it is the amount of new biomass that is produced, from whatever the standing stock of biomass might be, that is important for the next trophic level.
We can examine this further by constructing
a pyramid of energy, which shows rates of production rather than
standing crop. Once done, the figure for the ocean would have the characteristic
pyramid shape (see Figure 4c). Algal populations can double in a
few days, whereas the zooplankton that feed on them reproduce more slowly
and might double in numbers in a few months, and the fish feeding on zooplankton
might only reproduce once a year. Thus, a pyramid of energy takes into
account the turnover rate of the organisms, and can never be inverted.
Figure 4: Pyramids of numbers, biomass, and energy for the oceans.
We see that thinking about pyramids of energy and turnover time is similar to our discussions of residence time of elements. But here we are talking about the residence time of "energy". The residence time of energy is equal to the energy in biomass divided by the net productivity, Rt = (energy in biomass / net productivity). If we calculate the residence time of energy in the primary producers of various ecosystems, we find that the residence times range from about 20-25 years for forests (both tropical rainforests and boreal forests), down to ~3-5 years for grasslands, and finally down to only 10-15 days for lakes and oceans. This difference in residence time between aquatic and terrestrial ecosystems is reflected in the pyramids of biomass, as discussed above, and is also very important to consider in analyzing how these different ecosystems would respond to a disturbance or what scheme might best be used to manage the resources of the ecosystem.
We can start by looking at the Inputs and Outputs:
Inputs: NPP, calculated as
annual harvest. In a cropland NPP and annual harvest occur in the same
year. In forests, annual harvest can exceed annual NPP (for example, when
a forest is cut down the harvest is of many years of growth), but we can
still compute annual averages.
Units: We will use the Pg or Pedagram of organic matter (= 1015 g, = 109 metric tons, = 1 "gigaton") (1 metric ton = 1,000 kg).
Table 1: Surface area by type of cover and total
(from Atjay et al. 1979 and De Vooys 1979).
1. The Low Calculation: (See Table 2)
(a) Plant material directly consumed = 5 billion people X 2500 kcal/person/day X 0.2 (to convert kcal -- organic matter) = 0.91 Pg organic matter. If we assume that 17% of these calories derive from animal products, humans directly consume 0.76 Pg of plant matter. Estimate of human harvest of grains and other plant crops is 1.15 Pg annually. This implies loss, spoilage, or wastage of 0.39 Pg, or 34% of the total harvest.
2. The Intermediate Calculation: (See Table 3)
We add to the low calculation the amount of NPP co-opted by humans. This is:
Table 3: Intermediate calculation of NPP co-opted by humans
For the high estimate we now include both co-opted NPP and potential NPP lost as a consequence of human activities:
(a) Croplands are likely to be less productive than the natural systems they replace. If we use production estimates from savanna-grasslands, it looks like cropland production is less by 9 Pg.
Table 4: High calculation
of NPP co- opted by humans.
What can we conclude from the above analysis of the fate of net primary production in our world?
(a) Human use of marine productivity is relatively small. Moreover, although major fish stocks are heavily fished, and many coastal areas are severely polluted, human impact on the seas is less than on land.Controls on Ecosystem Function
Now that we have learned something about how ecosystems are put together and how materials and energy flow through ecosystems, we can better address the question of "what controls ecosystem function"? There are two dominant theories of the control of ecosystems. The first, called bottom-up control, states that it is the nutrient supply to the primary producers that ultimately controls how ecosystems function. If the nutrient supply is increased, the resulting increase in production of autotrophs is propagated through the food web and all of the other trophic levels will respond to the increased availability of food (energy and materials will cycle faster).
The second theory, called top-down control, states that predation and grazing by higher trophic levels on lower trophic levels ultimately controls ecosystem function. For example, if you have an increase in predators, that increase will result in fewer grazers, and that decrease in grazers will result in turn in more primary producers because fewer of them are being eaten by the grazers. Thus the control of population numbers and overall productivity "cascades" from the top levels of the food chain down to the bottom trophic levels.
So, which theory is correct? Well, as is often the case when there is a clear dichotomy to choose from, the answer lies somewhere in the middle. There is evidence from many ecosystem studies that BOTH controls are operating to some degree, but that NEITHER control is complete. For example, the "top-down" effect is often very strong at trophic levels near to the top predators, but the control weakens as you move further down the food chain. Similarly, the "bottom-up" effect of adding nutrients usually stimulates primary production, but the stimulation of secondary production further up the food chain is less strong or is absent.
Thus we find that both of these controls
are operating in any system at any time, and we must understand the relative
importance of each control in order to help us to predict how an ecosystem
will behave or change under different circumstances, such as in the face
of a changing climate.
All materials © the Regents of the University of Michigan unless noted otherwise.